Friction materials for use in brake systems for various vehicles, industrial machines, and so on are required to have high and stable coefficients of friction and thus excellent fade resistance, have excellent wear resistance, and have low aggressiveness against rotors. To meet these characteristics, use was made, as the friction materials, of resin compositions in which asbestos, an inorganic filler, an organic filler, and so on are compounded with a thermosetting resin (binder), such as a phenolic resin, for binding the above additives.
However, because asbestos has been confirmed to be carcinogenic and is easily ground into dust, its use is refrained owing to environmental health problems attributable to its inhalation at work. For this reason, there is proposed as an alternative a friction material in which a fibrous alkaline titanate, such as potassium titanate, is used as a friction modifier. Particularly, potassium titanate fibers are not carcinogenic unlike asbestos, do not damage rotors unlike metal fibers, and have excellent frictional properties, but most of conventional types of potassium titanate fibers have an average fiber diameter of 0.1 to 0.5 μm and an average fiber length of 10 to 20 μm, which are not included within a range of fibers recommended by the World Health Organization (WHO) (range of fibers except for WHO fibers as inhalable fibers, i.e., fibrous compounds having an average minor diameter of 3 μm or less, an average fiber length of 5 μm or more, and an aspect ratio of 3 or more). As a solution to this, potassium titanate having an amoeboid form is proposed in Patent Literature 1.
A fade phenomenon of a friction material is a phenomenon caused because organic components in the friction material are gasified with increasing temperature of the friction material, so that a gas layer is formed at the frictional interface with a disc. The fade resistance of the friction material can foe improved by preventing the formation of the gas layer at the frictional interface. To this end, it is useful to increase the porosity of the friction material to allow easy escape of the gas from the frictional interface. It is conceivable as a method for increasing the porosity of the friction material to adjust and set the forming pressure at a smaller value in the step of binding and forming a source material mixture. However, when the forming pressure is lowered, the strength and wear resistance of the friction material decreases, so that fractional properties cannot be obtained. To cope with this, in Patent Literature 2, hollow powder of an alkaline titanate is proposed which is formed of hollow bodies in each of which alkaline titanate particles having a rodlike, prismoidal, columnar, strip-shaped, granular and/or platy form are bound together.